1. Field of the Invention
The present invention relates to a semiconductor device including a level shifting circuit between circuit blocks using two different power supplies, particularly a level shifting circuit which inactivates a circuit block using one power supply at a time when a voltage of the other power supply abnormally drops.
2. Description of the Related Art
In general, an appropriate power supply voltage needs to be supplied to each circuit block in order to allow a plurality of circuit blocks (function blocks) to perform desired operations in an integrated circuit, and a multi-power supply has been used. A level shifting circuit for shifting a voltage level is used in an interface between the circuit blocks using two power supplies with different voltages.
In a most general method of shifting the level, an output signal having a second voltage level is obtained from an output terminal of a flip flop, which operates at the second voltage level (logic level), using a complimentary pair of signals having a first voltage level (logic level) as input signals of the flip flop. This type of level shifting circuit has various problems, and improvements have been attempted.
For example, when a first power supply voltage changes to a predetermined voltage level (first voltage level) from a GND level, a feedthrough current temporarily flows in a part of the flip flop. This feedthrough current can be suppressed, when a capability ratio of Nch to Pch transistor is increased, but this causes a problem that a size of the transistor increases. In Jpn. Pat. Appln. KOKAI Publication No. 2001-68978, transistors driven at a third voltage level which is an intermediate voltage between first and second voltage levels are inserted in two main current paths of the flip flop, and a voltage between gate and source of the cross-coupled transistors is lowered to reduce the feedthrough current.
By the way, the semiconductor device is controlled by an input signal from the outside. An amplitude of the input signal is limited to one voltage amplitude even when there is a circuit block using a plurality of power supply circuits, and the amplitude of a lowest first power supply voltage is used in many cases. At this time, the circuit block operating at a second voltage receives the input signal from the outside after passing through the level shifting circuit.
When there are a plurality of power supplies, all the power supply voltages do not necessarily constantly indicate a normal value. It is now assumed that a voltage value of a first power supply which supplies a first voltage level drops to such an extent that logic circuits such as an inverter do not operate, whereas only a second power supply which supplies a second voltage level indicates a defined output. The circuit block which operates from the second power supply receives a signal obtained by shifting the level of the input signal with the first voltage level through the level shifting circuit. Now, since the first power supply is in an unstable state before reaching the first voltage level, an output after the level shifting is also unstable, and the circuit block to which the output is supplied is not controlled by the input signal from the outside. However, the second power supply, which is in a stable state, has been supplied to the circuit block. Therefore, the circuit block is probably inadvertently activated. This abnormal operation has to be avoided especially in a case where current consumption of the circuit block using the second power supply is large.
In a case where the output voltage of the first power supply on a low voltage side drops to such an extent that the logic circuit is incapable of operating, the circuit block using the second power supply to which the state of the external input signal is indirectly transmitted via the level shifting circuit is brought into a non-controlled state. Accordingly, the circuit block using the second power supply is inadvertently active, and there is a possibility that a current is passed from the second power supply on a high voltage side.
Therefore, there has been a demand for realization of a semiconductor device which can prevent a circuit from being inadvertently active, the circuit using a second power supply voltage and being supplied with an input signal level-shifted from a first voltage level which is supplied from the first power supply, in a case where the first voltage level drops below a defined value.